Unless I'm missing something, Hasura requires much less effort on the back-end than what you're describing with Absinthe. Hasura runs in its own process (or processes; it doesn't have any state of its own so it can scale), and it can deliver events to your back-end via webhooks, so it doesn't matter what language you use for the back-end.

As for file uploads, it seems to me that the best way to do that is to have the front-end upload directly to your cloud storage service. Both S3 and Google Cloud Storage have a feature called signed URLs, where your back-end can create an object, grant the necessary permission, then give the client a temporary URL to upload to that object. Then just store the URL in the database.